Esophageal and CVP changes were measured simultaneously during mechanical ventilation in 12 patients with acute respiratory failure (ARF). The results of these measurements were different and showed no correlation. Values of transpulmonary pressure changes and calculated lung compliances correlated well, because of the higher airway pressure changes. It is concluded, therefore, that measurements of esophageal and CVP changes are equally well suited for these calculations. For practical purposes, there is no need to measure a representant of intrapleural pressure changes, because during mechanical ventilation total static compliance calculations can be used to monitor changes in lung compliance, provided the thoracic cage compliance is not reduced substantially and does not change during the course of the studies. Clinical awareness of factors influencing thoracic cage compliance is important. The difference in transpulmonary and transthoracic pressure relationships during mechanical ventilation and during spontaneous breathing is emphasized. In spontaneous breathing, intrapleural pressure changes are determined primarily by the elastic properties of the lungs; in mechanical ventilation, on the other hand, by the elastic properties of the thoracic cage.